Excel Displaying Results Incorrectly

I have been using PHPExcel for quite some time now for a client of mine. I have a report written in php that he runs and he often needs to export it to excel, therefore PHPExcel was the ideal solution.

However the reporting database has grown, and therefore so has excel. I have noticed that when excel creates a workbook with over 150 rows, its starts to go a bick haywire. Basically misplacing columns. there are 11 columns and after row 150ish it starts
to put column 11 in column 1, column 10 in column 11, etc..

If anyone can help me with this I would be very grateful, as I am getting a lot of pressure from my client and I cannot seem to figure a solution.